1. Field of the Invention
The present invention relates generally to heating human or animal tissue (hyperthermia) and more particularly to electromagnetic radiation (EMR) apparatus for heating local areas within such living body tissue.
2. Background Information
As is generally known, death, or necrosis, of living tissue cells occurs at temperatures elevated above a normal cell temperature. Further, the death rate of such heated tissue is a function of both the temperature to which it is heated and the duration for which the tissue is held at such temperatures.
It is also well known that the elevation of temperature of living tissue can be produced with electromagnetic energy at frequencies greater than about 10 KHz.
It has been reported that some types of malignant cells may be necrotized by heating them to a temperature which is slightly below the temperature injurious to most normal cells. In addition, some types of malignant cells may be selectively heated and necrosed by hyperthermia techniques because masses of these malignant cells typically have considerably poorer blood flow and thus poorer heat dissipation properties than does the surrounding normal tissue. As a result, when normal tissue containing such malignant masses is heated by EMR (electromagnetic radiation), the resultant temperature of the malignant mass may be substantially above that of surrounding healthy cells.
Although some disagreement exists regarding exact temperatures, most malignant cells have a relatively limited temperature range in which hyperthermia is effective in causing necrosis. Below a threshold temperature of about 41.5.degree. C. (106.7.degree. F.) insubstantial thermal damage occurs even in those types of malignancies which have a greater sensitivity to temperature than do normal cells. In fact, at temperatures just below this threshold, growth of some types of malignancies may be stimulated. At temperatures within or above 43.degree. to 45.degree. C. (109.4.degree. to 113.degree. F.) thermal damage to most normal cells occur. A discussion of hyperthermia in the treatment of cancer is contained in "Physical Hyperthermia and Cancer Therapy" by J. Gordon Short and Paul F. Turner in the Proceedings of the IEEE, Vol. 68, No. 1, January, 1980 herein incorporated by reference.
Typically, EMR heating of body tissue is accomplished by holding an EMR radiator, or applicator, adjacent to, or against, exterior portions of a body, the EMR then penetrating and heating subsurface portions of tissue. However, significant amounts of energy are absorbed by surface or epidermis layers which may have to be cooled in order to prevent damage thereto by overheating.
The amount of penetration, or the depth of which EMR causes effective heating, is dependent upon the frequency of radiation.
For example, in accordance with an article by A. W. Guy, et al, published in proceedings of the IEEE, volume 63, No. 1, January, 1974 entitled "Therapeutic Application of Electromagnetic Power", the depth of penetration in the human muscle and fat at 100 MHz is 6.66 cm (2.62 inches) and 60.4 cm (23.8 inches), respectively, while at 915 MHz the depth of penetration is only 3.04 cm (1.2 inches) and 17.7 cm (6.97 inches), respectively.
In general, the lower the EMR frequency, the larger the applicator must be in order to effectively radiate electromagnetic energy into the tissue and, as a result, applicators for radiating electromagnetic energy below one gigahertz tend to be large in size and cumbersome to handle. Additionally, such applicators are not configured to selectively heat tumors of various sizes and shapes located well beneath the surface layers of the body being irradiated. Further, tumors, or other selected areas, shielded by a layer of boney tissue such as a skull, are different to effectively heat with externally applied EMR.
Invasive EMR applicators, that is, radiators which can be inserted into body tissue to levels adjacent malignant tumors, or other localized growths, for selective heating thereof, may cause nonuniform heating, or "hot spotting" at or near the surface of such applicators because of nonuniform field distributions. Such unwanted "hot spotting" is more likely to cause serious overheating when such invasive applications are operated at higher power levels in order to heat large localized growths using a single applicator. Such growths may be many times the size of the radiating area of an invasive type applicator.
An example of invasive EMR applicators are disclosed in U.S. Pat. No. 4,448,198 entitled "Invasive Hyperthermia Apparatus and Method" which discloses the application of several invasive type applicators and a method of using the apparatus to effectively heat relatively large localized areas within living body tissue, without significant hot spotting at or about the applicators.
The localized areas included those located well beneath surface layers of the body tissue. According to the disclosed method, the heating of these local regions within living body tissue were accomplished with a radiation source providing electromagnetic radiation to several applicators. Each of the applicators were adapted for insertion into the body tissue and for radiation electromagnetic energy therein.
In addition to the applicators, several invasive temperature probes were disclosed for monitoring temperatures within a target growth and also the normal tissue just outside the target growth.
It is the object of the present invention to provide an applicator for insertion into body tissue for radiation of electromagnetic radiation and for the detection of temperature therein.
It is a further object of the present invention to provide a system to provide radiation to the applicators in accordance with the sensed temperatures to control the heating of localized areas of body tissue.
It is still further an object of this invention to provide a system for controlling radiation provided to localized areas of body tissue by calibrating the applicator temperature sensor to determine the portion of measured temperature resulting from the heating of the applicator.